The present invention relates to improvements in a multi-blade ditching machine for use in burying submarine cables and the like under the sea floor or lifting them up for repairing.
To protect submarine cables or the like from fishing tools, it has been the practice in many countries of the world to bury a cable and the like in the sea floor and to pull them out for repairing when faults are detected in them. For this purpose, ditching machines which ditch the soil and/or sediment of the sea floor to a desired depth have been used and such ditching machines are called cable-buriers or cable-searchers depending on the purpose thereof.
Although the structure of a cable-burier is somewhat different from that of a cable-searcher due to the difference in their purposes, the essential ditching portions of both kinds of machine are similar to each other, and water jets or plows are used for ditching trenches on the sea floor. When it comes to the plow ditching portions, two types have been used, i.e., a single-blade type ditching portion and a multi-blade type ditching portion.
A ditching machine (or a cable-burier) with the multi-blade type ditching portion is described in U.S. Pat. No. 4,312,144, and Japanese Pat. No. 1130568. FIG. 4 is side elevation view of the latter multi-blade ditching machine illustrating its structure. FIG. 5 is a view taken along the line B--B of the multi-blade ditching machine shown in FIG. 4. In FIGS. 4 and 5, the reference numeral 1 stands for a stabilizing wing, 2 stands for a ditching portion, 3 stands for a center leading body, 3a stands for its main body, 3b stands for its wedge portion, 4 stands for a joint connecting the stabilizing wing to the ditching portion, 5.sub.1 through 5.sub.7 stand for ditching blades and soil-pushing blades. In this embodiment, there are 7 blades in the ditching portion. (i.e.,n=7)
The multi-blade ditching machine has a stabilizing wing 1 at its front position which provides the machine stability in the horizontal plane, to which the ditching portion 2, which ditches a trench on the sea floor, is connected by joint 4 which makes the ditching portion moveable only in the vertical plane.
To lead a cable and/or a repeater in the bottom of a trench thus excavated, the ditching portion 2 has a center leading body 3 at its center back position which serves as a passage for leading them.
The structure of the ditching portion 2 is composed in such a way that the more forwardly positioned ditching blade have a wider blades width and smaller blade depth, and the depth from the sea floor to a ditching blade gradually increases as its position moves in the rear direction. Thus, ditching blades 5.sub.1 through 5.sub.7 , are disposed along the lower side of center body 3.
On the upper side of each of the ditching blades 5.sub.1 through 5.sub.7, a soil-pushing blade is disposed for pushing the excavated soil and/or sand outwardly and both edges of the soil-pushing blade are cut slant-wise so that they open upwardly. Each soil-pushing blade of the forward blades (5.sub.1 through 5.sub.4) pushes the excavated soil and/or sand away over the sea floor, and each soil-pushing blade of the rear blades (5.sub.5 through 5.sub.7) pushes the excavated soil and/or sand away from the ditch just dug. As an example, consider the sixth blade (which is a rear blade). In pushing away the soil and/or sand excavated by the sixth blade 5.sub.6, the inverse trapezoid shaped space created by passing of the preceding blade, i.e., blade 5.sub.5 (see FIG. 5) is utilized as its passage along the flow of it. (this will be called a sand-pushing passage.) Said slantings of a soil-pushing blade (that is, the shape of an inverse trapezoid) prevent the excavated soil and/or sand from falling into the trench just ditched, and serve to reduce the towing force of the ditching machine.
Generally speaking, when the sea floor is composed of sand, the ditching resistance of a ditching blade is proportional to the square of the ditching depth of the blade. Therefore, the ditching resistance of a multi-blade ditching machine which repeats shallow ditchings to a desired depth can be reduced by a factor one-sixth as compared with that of a single-blade ditching machine.
As the ditching resistance of a multi-blade ditching machine is very small, a little amount of soil-pushing resistance which might be negligible in case of a single-blade ditching machine brings about a problem from a view point of reducing the total towing resistance of the machine further.
It is true in common to all sorts of ditching machines that the existence of the center body 3 whose width is relatively wide for leading repeaters and the like makes the soil-pushing resistance large.
On one hand, this soil-pushing resistance enhances the towing resistance of the machine, and on the other it provides an undesireable cause to make the ditching depth of the machine smaller by pushing up the center body's lower side 3b.
Therefore, to make the flow of the pushed-away soil as smooth as possible, the center body has a wedge portion on its lower side, being constructed by two tapering planes on right and left, each of which makes an angle of 45.degree. with respect to the vertical plane, i.e., the vertex of the wedge is 90.degree.. (see FIG. 3 and FIG. 5)
A cable ship or work ship tows said multi-blade ditching machine through a towing wire (not shown in the figures) that is connected to the head part of the stabilizing wing and the sea floor is ditched as a result.
The submarine cables and repeaters drawn out from the work ship are led to the ditch bottom by way of the center body 3 and are pushed to the ditch bottom by the weight of the pushing roller 6. The pushing roller 6 is moveable along the guide 8. The reference numeral 7 stands for a frame that is rotatable around a pivot located at the other end of the pushing roller. A caster, denoted by 9, is disposed for ease of carrying the machine on a work ship, decending it into the sea floor, and refloating it from the sea floor.
A multi-blade ditching machine of the structure mentioned above, however, has the following problems.
As shown in FIG. 5, the space of the sand-pushing passage of the sixth blade is limited by the reverse side of the fifth blade 5.sub.5 and the surface of the sixth blade 5.sub.6 for its up-and-down boundaries, and for its right-and-left boundaries, as they are symmetrical with respect to the vertical plane at center, consider as an example the left-half space. The leftmost boundary is the side wall of sand whose surface is slanted by an angle of 15.degree. (if cut by a plane perpendicular to the moving direction, this is 16.degree.) with respect to the vertical plane, and on the right-hand side where the center body exists, the tapering surface 3b of the wedge portion which is slanted by an angle 45.degree. with respect to the vertical plane limits the half space.
As to the opposed two planes limiting the up-and-down boundaries of the space, care has been taken in the design so that the pushed-away soil and/or sand does not touch the reverse side of the preceding blade, i. e., blade 5.sub.5 by choosing an appropriate distance between the fifth blade 5.sub.5 and the sixth blade 5.sub.6.
Meanwhile, as to the opposed two planes of the right-and-left boundaries, i.e., the surface of the side wall of sand and the tapering surface 3b of the wedge portion of the center body the difference between their slanted angles with respect to the vertical plane is as large as 15.degree.-45.degree.=-30.degree. (where the negative sign means "tapering off") and so the space functions as a sand-pushing passage of "tapering off" to the pushed-away soil rising along the blade.
In particular, when the sea floor is composed of sand, the flow of the excavated sand passing through this sort of a "tapering off" sand-pushing passage tends to be clogged, and this causes the soil-pushing resistance to increase remarkablly. At the same time, the upward-directed force component due to the flow reduces the ditching depth unreasonablly by pushing up the tail part of the ditching portion 2.
Therefore, in designing a ditching machine or the like, care should be taken to avoid the "tapering off" by paying due attention to the up-and-down, right-and-left boundaries of a cross section of the passage. In particular, when the sea floor is composed of sand, because the characteristic of sand in such a flow is entirely different from that of soil or clay, and it is in the field of gravity, the most effective way to overcome the obstruction of a sand-pushing passage is to avoid the "tapering off" of the right-and-left boundaries of a cross section of the passage. (the tapering off of a sand-pushing passage will mainly mean that of the length between the right-and-left boundaries of a cross section.)